DON'T PUT GUINEAPIGS ON SAWDUST! I lost my first one, the most fantastic g'pig, through sawdust - he got a large splinter in the back of his tongue and despite going to the vets (who are mostly not that hot with g'pigs) he got septicaemia and died - I was heartbroken! (aged 28 at the time, didn't have any children then). Keep them on straw and hay. Or shredded newspaper but their feet get inky. Not shredded office paper as it's too sharp for their feet.
Guineapigs also are (I think) the only other mammal (other than us) that cannot make their own Vitamin C so it is important to ensure that they get daily fresh fruit/veg that contains Vitamin C, or dried g'pig food that has it added. Rabbit food is not a good option for g'pigs as it doesn't have Vit C added and can have something else added (I forget what) that is bad for g'pigs.
Good veg/fruit for g'pigs: greens, kale, tomato, cucumber, carrot (whole, need it to keep their teeth down), celery, broccoli, apple, banana (occasional treat).
Not so good: spinach, lettuce (oxalates can cause kidney stones in g'pis), parsnip (many don't like the flavour)
Lots of hay and occasional fresh grass - not too much or they can get diarrhoea (mind you, mine were all housepigs so didn't get out much!).
Water bottle essential - some will drink lots, some will drink almost nothing but so long as they have the daily fresh fruit/veg that doesn't matter.
G'pigs need to have their claws clipped regularly - for this reason I would advise getting one with pale claws, not black ones, so you can see where the quick is!
G'pigs can be more docile than rabbits but that depends hugely on the g'pig and the rabbit! As cavycare says, do not keep them together - if the rabbit gets scared and kicks out and hits the g'pig, it is likely to have internal damage which could be fatal. NB: G'PIGS DON'T BOUNCE - do not drop them, their legs are too short to withstand the shock and it can kill them. They can also freeze to death so must be protected in harsh winter weather.
If your g'pig gets sick, try {{http://www.cavy-vet.co.uk/ this site]] for an appropriate vet, or try the Cambridge Cavy Trust
Sorry if TMI - g'pigs are fab but have slightly different needs from rabbits.